FIG. 1 of the accompanying drawings illustrates the stack structure of a typical liquid crystal display (LCD) module of small size, for example for a mobile phone or PDA device. The display comprises a flat transmissive spatial light modulator (SLM) in the form of an LCD panel having input and output polarisers on its bottom and top sides. The rest of the structure is generally regarded as the backlight system, as follows. A light source (for example an LED or Laser) emits light, which is coupled into a light guide and distributed across the back of the display by way of total internal reflection (TIR) in such a way that if no scattering structures were present the light would travel until it reached the end of the light guide. Within the light guide there are multiple scattering structures that extract the light from the light guide to illuminate the LCD panel by disrupting the TIR conditions at the surface of the light guide on which they are located, hence allowing the light to pass through the air light guide interface. These scattering features may be located on either the top or bottom major light guide surfaces. The density of the light scattering features may increase with distance from the light source to maintain a uniform rate of extraction of the light along the length of the light guide. As light is extracted both down and up from the light guide, a reflecting film is placed beneath the light guide to improve the efficiency of the backlight. There are also some optical films between the light guide and the LCD panel, placed to give better illumination uniformity over the display area and to enhance brightness within a given viewing angle range. These films typically consist of diffuser layers and prism films that enhance the central brightness of the backlight. The form of these structures is well known in the prior art and will not be discussed further here.
The form of the features that extract the light can take many forms. The form that this extraction takes can determine the angular profile of the lightguide emission which can then be diffused or utilized in some manner. For example, backlights that require the lightguide to produce collimated emission, the form of the emission will be very sensitive to the range of angles of the light in the lightguide. Other potential extraction, such as sub-wavelength, diffractive or holographic features, will also be dependent on the angle range for the quality of their extraction profile.
In order to minimize loss in the lightguide within the backlight, it is generally necessary to extract a large fraction of the light before it reaches the other end. Reflection and a second pass of light are generally inefficient and introduce substantial non-uniformity.
Extraction features that out-couple light using sub-wavelength or photonic features in order to extract light from the lightguide are necessarily much smaller than typical non-diffractive features that are needed to maintain good uniformity on the lightguide in a single pass. Because of this, the efficiency of extraction is generally low, especially where the features are only refractive index gratings. In addition, patterning the extraction means a substantial fraction of the lightguide surface (especially near the light sources) is not used for extraction reducing further the overall extraction efficiency. This means it is often very difficult to get full efficient extraction using only diffractive features.
US 2006/0285185 (Samsung Electronics Co.) describes a holographic grating whose amplitude increases away from the light sources to enhance extraction efficiency. Although not reported, overall extraction efficiency is expected to be poor especially for small area backlights.
US 2006/0187677 (Parrika et. al.) describes a diffractive backlight with a grating whose duty cycle (or fill factor) increases further from the light emitting devices to increase light extraction. The efficiency of this design is limited by the period of the grating, the smaller the period the less the duty cycle that can be filled. Also overall extraction efficiency is expected to be poor especially for small area backlights.
US 2005/0111814 (Taiwan Nano Electrico-Optical Technology Co.) describes a diffractive backlight with longitudinal diffraction elements only. The size, density and shape of the diffraction elements vary along the backlight to improve uniformity. Total extraction efficiency is expected to be very poor.
US 2005/0052732 (Hon-Hai Precision Industry Co.) describes a backlight with a number of diffraction units of different efficiency depending on the orientation of the grooves with respect to the direction of light. This design might suffer from angular non-uniformities. Also its total extraction efficiency might be poor.
U.S. Pat. No. 7,253,799 (Samsung Electronics Co.) describes a backlight system with a diffraction grating whose frequency, shape and amplitude remains constant. A mirror mounted at the end of the light guiding plate opposite to the light sources reflects non-extracted light backwards for recycling. Display will appear brighter close to the two ends of the backlight and dimmer towards its middle and there will be losses on reflection.
U.S. Pat. No. 6,773,126 (Oy Modilis Ltd.) describes diffraction elements printed on a backlight with varying extraction efficiency depending on their orientation with respect to the light propagation. Uniformity is achieved by placing the “weaker” diffraction elements closer to the light sources and the “stronger” elements further away from the light sources. Angular non-uniformities will occur with this design and total extraction efficiency will be poor for small area backlights.